As a spherical annular seal member used in a spherical pipe joint for an automobile exhaust pipe, one disclosed in, for example, Japanese Patent Application Laid-Open No. 76759/1979 is known. The seal member disclosed in this publication is heat resistant, excels in affinity with a mating member, and has remarkably improved impact strength, but has a drawback in that the seal member often generates abnormal noise when it undergoes friction under dry frictional conditions. The drawback of this seal member is conceivably attributable to, among others, the fact that there is a large difference between the coefficient of static friction and the coefficient of dynamic friction of a heat-resistant material (such as expanded graphite) for forming the seal member, and to the fact that the seal member constituted by this heat-resistant material exhibits negative resistance with respect to the sliding velocity or speed.
Therefore, to overcome the above-described drawback, the present applicant proposed a seal member disclosed in Japanese Patent Application No. 300551/1992 (Japanese Patent Application Laid-Open No. 123362/1994). This seal member excels in the sealing characteristic without generating abnormal frictional noise in the sliding on a mating member, and satisfies the performance required of a seal member.
However, with respect to this proposed seal member as well, a new problem has been presented which is attributable to, among others, the improvement in recent years of the performance of automobile engines. That is, the conventional seal members are unable to satisfy the conditions of use in the light of heat resistance, owing to an increase in the exhaust-gas temperature due to the improved performance of the automobile engines, or owing to an increase in the exhaust-gas temperature attributable to the fact that the spherical pipe joint is located closer to the engine side in a case where the spherical pipe joint is disposed in the vicinity of an outlet (manifold) of the exhaust gases, for the purpose of improving the noise, vibration and harshness (NVH) characteristics of an automobile. Thus, there has been a compelling need for improvement of the heat resistance of the seal member itself.
With respect to the above-described newly presented problem, the present applicant proposed spherical annular seal members and methods of manufacturing the same in which heat resistance is improved through Japanese Patent Application No. 186783/1996 (Japanese Patent Application Laid-Open No. 9396/1998) and Japanese Patent Application No. 186784/1996 (Japanese Patent Application Laid-Open No. 9397/1998) (hereinafter, these applications for patent will be referred to as the prior art).
The spherical annular seal members in accordance with the above-described prior art are capable of suppressing oxidation and wear to low levels, do not generate abnormal frictional noise, excel in sealing characteristics, and are capable of satisfying their functions as seal members even under high temperatures of 600° C. to 700° C. However, in the case of these spherical annular seal members, since a heat-resistant sheet member, e.g., a heat-resistant sheet member having on the surfaces of an expanded graphite sheet a heat-resistant coating formed of heat-resistant materials, is used in the manufacturing methods, the flexibility inherent in the expanded graphite sheet is sacrificed. In consequence, there are possibilities that the cracking, breakage, and the like of the heat-resistant coating and, hence, the breakage and the like of the heat-resistant sheet member can often occur in the bending operation and the like involved in the manufacturing process. Thus, it was found that there is room for improvement in terms of the material yield, and that the elimination of the drawback of the material yield has the advantage of shortening the process of manufacturing the spherical annular seal member, leading to the reduction of the manufacturing cost.
The present invention has been devised in view of the above-described aspects, and its object is to provide a spherical annular seal member which exhibits performance equivalent to that of the spherical annular seal members of the above-described prior art in that the spherical annular seal member has heat resistance (resistance to oxidation and wear), does not generate abnormal frictional noise, and excels in the sealing characteristic even under high temperatures of 600° C. to 700° C., and which, in its manufacturing method, is capable of overcoming the drawback of the material yield of the heat-resistant sheet member and of lowering the manufacturing cost, as well as a method of manufacturing the same.